1. Field of the Invention
The present invention is directed to a double echo sequence and to a magnetic resonance apparatus for the implementation of the double echo sequence.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static, basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus also has a radio-frequency system that radiates radio-frequency signals into the examination subject for triggering magnetic resonance signals, and picks up the generated magnetic resonance signals, from which magnetic resonance images are produced.
In an image acquisition mode, the magnetic resonance apparatus is controlled on the basis of a sequence executed in a central control system of the magnetic resonance apparatus such that, among other things, the gradient fields are switched at the proper time with defined intensity, the radio-frequency signals are emitted at the proper time, and the magnetic resonance signals are picked up at the proper time.
A multitude of different sequences are known in magnetic resonance technology, with magnetic resonance images having different imaging properties being able to be generated with the various sequences. A double echo sequence includes a gradient echo and a spin echo, and is employed often in orthopedics, for example for generating images of the knee of a patient, and is known under the acronym DESS (Double Echo Steady State). Further details about the double echo sequence are disclosed, for example, by European Application 0 288 861 and in the article by H. Bruder et al., “A New Steady-State Imaging Sequence for Simultaneous Acquisition of Two MR Images with Clearly Different Contrasts”, Magnetic resonance in Medicine 7 (1988) pages 35-42.
The double echo sequence corresponds to a combination of a refocused gradient echo sequence, which is known, among other things, by the acronym FISP (Fast Imaging with Steady State Precision and a sequence wherein a spin echo based on a refocusing partial effect of each radio-frequency signal is utilized and that is known, among other things, by the acronym PSIF. The acronym PSIF is a reversal of the order of the letters of the acronym FISP since the pattern of radio-frequency signal and gradient of the PSIF sequence approximately corresponds to a FISP sequence implemented in reverse. The FISP sequence is distinguished among gradient sequences by comparatively strong magnetic resonance signals since the longitudinal as well as the transverse magnetization, held in an equilibrium state are utilized for imaging. Magnetic resonance images having a dominant T1/T2 contrast can thereby be generated with the FISP sequence. By contrast, the magnetic resonance images generated with the PSIF sequence exhibit a strong T2 weighting. The double echo sequence supplies two three-dimensional (for example) datasets, one of the datasets belonging to the gradient echoes and one to the spin echoes. A combination of the two datasets then yields a corresponding dataset, and thus a magnetic resonance image of the double echo sequence.
The article by E. M. Haacke et al., “A Comprehensive Technical Review of Short TR, Fast, Magnetic resonance Imaging”, Reviews of Magnetic resonance in Medicine, vol. 3, no. 2, 1991, pages 53-170, a variant of the PSIF sequence, which is called ROAST (Resonant Offset Averaging in the STeady State) wherein at least one sub-region of a region to be imaged that flows during the image acquisition, for example slowly flowing blood, can experience such a pronounced dephasing that this sub-region supplies to signal contribution to the magnetic resonance image.
It is also known that a number of artifacts can appear in the magnetic resonance image given an imaged region wherein flow phenomena, for example an arterial blood flow, occur. The two most striking effects are a reduction of the magnetic resonance signal as a consequence of an incoherent addition of phases of the individual spins within a picture element, and the formation of a number of ghost images as a consequence of the pulsating blood flow during the heart cycle. Since both effects exhibit a proportionality to the first time moment of the gradient fields that are utilized, the aforementioned artifacts can be reduced by means of a flow compensation, by setting the first moment to zero. For example, a triple gradient pulse instead of a bipolar gradient pulse is utilized in a slice selection direction and frequency coding direction. For this purpose, for example, a unipolar gradient pulse is replaced by a bipolar gradient pulse in the phase-coding direction. The reason for the differences in the flow compensation between the slice selection gradient and the frequency-coding gradient between the slice selection gradient and the phase-coding gradient is that the phase coding is configured for producing a finite 0th moment, in contrast to which the frequency coding and the slice selection are not configured in this manner, but instead are intentionally designed usually as bipolar gradient pulses without flow compensation. This flow compensation is therefore also referred to in the literature as gradient moment rephasing. gradient motion refocusing or gradient moment cancellation. This is explained in greater detail in the article by L. R. Frank et al., “Elimination of Oblique Flow Artifacts in Magnetic resonance Imaging”, Magnetic Resonance in Medicine 25 (1992), pages 299-307. The topic of flow compensation is addressed in the article by G. A. Laub et al., “MR Angiography with Gradient Motion Refocusing”, Journal of Computer Assisted Tomography 12 (3), 1988, pages 377-382.